1. Wireless Mesh Networks
Anatolij Zubow
WLAN – Overview

2. Contents IEEE 802.11 WLAN architecture Management frames
Basic routing example
IAPP and mobility management
Basic frame structure
MAC header structure
Usage of MAC address fields
Management frames
Some IEEE standard amendments

3. IEEE 802.11 WLAN architecture
defines two BSS (Basic Service Set) options:
Infrastructure BSS
Independent BSS
(Ad-Hoc network)
wired LAN AP

4. Infrastructure BSS
This is by far the most common way of implementing WLANs.
The base stations connected to the wired infrastructure are called access points (AP).
Wireless stations in an Infrastructure BSS must always communicate via the AP (never directly).
Before stations can use the BSS: Association.
Infrastructure BSS
wired LAN AP

5. Independent BSS Mainly of interest for military applications.
No access point is required, stations can communicate directly.
Efficient routing of packets is not a trivial problem
(routing is not a task of ).
Independent BSS
(Ad-Hoc network)

6. Extended Service Set (ESS)
This is a larger WLAN network consisting of a number of BSS networks interconnected via a common backbone AP AP AP
supports link-layer mobility within an ESS (but not outside the ESS)

7. Distribution System (DS)
This is the mechanism by which APs and other nodes in the wired IP subnetwork communicate with each other.
Distribution System (DS)
Router
External network (LAN or Internet) AP AP
This communication, using the Inter-Access Point Protocol (IAPP), is essential for link-layer mobility (→ stations can seamlessly move between different BSS networks).

8. Distribution system (2)
For instance, when a wireless station moves from one BSS to another, all nodes must update their databases, so that the DS can distribute packets via the correct AP.
Distribution System (DS)
Router
AP 1
AP 2
AP 1, AP 2 and router: update your databases!
Packets for this WS will now be routed via AP 2. WS
WS moves to another BSS

9. Distribution System (DS)
Basic routing example
When WS associates with AP 2, the router in charge of the IP subnet addressing obtains an IP address from the DHCP (Dynamic Host Configuration Protocol) server.
External network (LAN or Internet)
Distribution System (DS)
Router
AP 1
AP 2 2 1
Association
Fetch IP address 1
DHCP
Server 2 WS

10. Basic routing example (2)
The router must maintain binding between this IP address and the MAC address of the wireless station.
External network (LAN or Internet)
Distribution System (DS)
Router 
00:90:4B:00:0C:72
AP 1
AP 2
00:90:4B:00:0C:72 WS

11. Basic routing example (3)
The globally unique MAC address of the wireless station is used for routing the packets within the IP subnetwork (DS + attached BSS networks).
External network (LAN or Internet)
Distribution System (DS)
Router 
00:90:4B:00:0C:72
AP 1
AP 2
00:90:4B:00:0C:72 WS

12. Basic routing example (4)
The router must also know (and use) the MAC address of the access point via which the packets must be routed. For this purpose, a special protocol (IAPP) is needed!
External network (LAN or Internet)
Distribution System (DS)
Router
00:03:76:BC:0D:12 
00:90:4B:00:0C:72
00:03:76:BC:0D:12
AP 1
AP 2
00:90:4B:00:0C:72 WS

13. IAPP (Inter-Access Point Protocol)
IAPP (defined in IEEE f) offers mobility in the Data link layer (within an ESS = Extended Service Set).
External network (LAN or Internet)
Distribution System (DS)
Router
AP 1
AP 2
AP 3 1 2
IAPP: APs must be able to communicate with each other when the station moves around in the WLAN

14. In addition to IAPP …
IAPP alone is not sufficient to enable seamless handovers in a WLAN. The stations must be able to measure the signal strengths from surrounding APs and decide when and to which AP a handover should be performed (no standardised solutions are available for this operation).
In networks, a handover means reassociating with the new AP. There may be two kinds of problems:
will handover work when APs are from different vendors?
will handover work together with security solutions?

15. IEEE 802.11 frame structure : TCP/IP protocol suite (usually) : IP
IP packet IP
IEEE 802 H
LLC payload
LLC
MAC H
MSDU (MAC SDU)
MAC
MPDU (MAC Protocol Data Unit)
PHY H
PSDU (PLCP Service Data Unit)
PHY
PPDU (PLCP Protocol Data Unit)

16. SDU (Service Data Unit) is sent between protocol layers
PDU vs. SDU
Payload of a PDU in layer N = SDU to/from the layer N+1 : :
SDU (Service Data Unit) is sent between protocol layers IP IP
LLC
LLC
MAC
MAC
PDU (Protocol Data Unit) is sent between network nodes (in a specific protocol layer)
PHY
PHY

17. MPDU (MAC Protocol Data Unit)
MAC header structure
MPDU (MAC Protocol Data Unit)
Addr 1
Addr 2
Addr 3
Addr 4 (optional)
MAC payload
FCS
Duration field (contains NAV value)
Sequence Control field (numbering of frames modulo 4096)
One byte (eight bits)
Frame Control field (type of frame & various flag bits)

18. Content of Frame Control field
One bit
Protocol
Type …
Subt. of frame 1 2 3 4 5 6 7 8
Protocol: Indicates IEEE MAC
Type:
00 (Management frames)
01 (Control frames)
10 (Data frames)
Subtype of frame: Describes type of management, control, or data frame in more detail (e.g. ACK => 1101)

19. Flags in Frame Control field
One bit
Protocol
Type …
Subt. of frame 1 2 3 4 5 6 7 8
1: Bit is set if frame is sent to AP
2: Bit is set if frame is sent from AP
3: Used in fragmentation
4: Bit is set if frame is retransmitted
5: Power management bit (power saving operation)
6: More data bit (power-saving operation)
7: Bit is set if WEP is used
8: Strict ordering of frames is required

20. Usage of MAC address fields
MPDU (MAC Protocol Data Unit)
Addr 1
Addr 2
Addr 3
Addr 4
Address 1: Receiver (wireless station or AP)
Address 2: Sender (wireless station or AP)
Address 3: Ultimate source/destination (router in DS)
Address 4:
Only used in Wireless Bridge solutions:
LAN
LAN AP AP

21. Direction: AP  wireless station
Addr 1
Addr 2
Addr 3
Addr 1: Receiver (wireless station)
Addr 2: Transmitter = BSSID (AP)
Addr 3: Ultimate source (router)
Router 3 2
BSSID: MAC address of AP
SSID: Alphanumeric name
of AP (or BSS) AP 1

22. MAC addressing example
Frames to the WS must also include the MAC address of the ”ultimate source” to which return frames should be routed (then ”ultimate destination”).
External network
Distribution System (DS)
Router
00:03:76:BC:0D:12
00:20:34:B2:C4:10
AP 1
AP 2 2 3
00:90:4B:00:0C:72 WS 1

23. Direction: Wireless station  AP
Addr 1
Addr 2
Addr 3
Addr 1: Receiver = BSSID (AP)
Addr 2: Transmitter (wireless station)
Addr 3: Ultimate destination (router)
Router 3 1 AP 2

24. Management frames
In addition to the data frames (containing the user data to be transported over the network) and control frames (e.g. acknowledgements), there are a number of management frames.
Note that these management frames compete for access to the medium in equal terms (using CSMA/CA) with the data and control frames.
Some of these management frames are presented on the following slides.

25. Beacon frames
Beacon frames are broadcast (meaning that all stations shall receive them and read the information) at regular intervals from the Access Point. These frames contain (among others) the following information:
Timestamp (8 bytes) is necessary, so that stations can synchronise to the network
Beacon interval (2 bytes) in milliseconds
Capability info (2 bytes) advertises network capabilities
SSID ( bytes), alphanumeric “network name”
The channel number used by the network (optional).

26. Probe request & response frames
A probe request frame is transmitted from a wireless station during active scanning. Access points within reach respond by sending probe response frames.
Probe request frames contain the following information:
SSID ( bytes), alphanumeric “network name”
Bit rates supported by the station. This is used by APs to see if the station can be permitted to join the network.
Probe response frames actually contain the same kind of “network information” as beacon frames.

27. Association request & response frames
Before a station can join an network, it must send an association request frame. The AP responds with an association response frame.
Association request frames contain (among others):
SSID, capability info, bit rates supported.
Association response frames contain (among others):
Capability info, bit rates supported
Status code (success or failure with failure cause)
Association ID (used for various purposes)

28. Passive and active scanning
Wireless stations can find out about networks by using passive or active scanning.
During passive scanning, the station searches beacon frames, moving from channel to channel through the complete channel set (802.11b => 13 channels).
During active scanning, the station selects Channel 1 and sends a probe request frame. If no probe response frame is received within a certain time, the station moves to Channel 2 and sends a probe request frame, and so on.

29. Station connecting to a WLAN
When a station moves into the coverage area of a WLAN, the following procedures take place: 1)
Scanning: the station searches for a suitable channel over which subsequent communication takes place 2)
Association: the station associates with an AP 3)
IP address allocation: the station gets an IP address, for instance from a DHCP server 4)
Authentication: only if this security option is required.

30. Handover to another AP
When a station has noticed that the radio connection to another AP is a better than the existing connection: 1)
Reassociation: the station associates with another AP 2)
No new IP address is needed; however, the WLAN must be able to route downlink traffic via the new AP 3)
Authentication: this security option, if required, will result in a substantially increased handover delay (complete procedure sequence: deauthentication, disassociation, reassociation, authentication).

31. Some IEEE 802.11 standard amendmentsf
IAPP e
QoS i
Security
basic protocol
MAC layer h
DFS/TCP d
Scanning a
OFDM 5GHz b
DSSS 2.4GHz g
OFDM 2.4GHz
Physical layer

32. Resources
HUT Communications Laboratory, “Wireless Personal, Local, Metropolitan and Wide Area Networks”